A semiconductor device array contains individual integrated circuits or semiconductor packages. Contacts of the semiconductor packages are exposed at the bottom of the semiconductor array. The contacts are typically made of copper. To prevent oxidation on the copper surface, the contacts are plated with a lead finished material, such as matte tin (Sn), using electroplating. As a result, the bottom surfaces of the contacts are thereafter tin plated.
Singulation is a process of separating each semiconductor package from a molded sheet. Dicing or sawing is a process that singulates the semiconductor array into individual or singulated semiconductor packages. Conventionally, the electroplated semiconductor array is diced into singulated semiconductor packages to be shipped to customers for assembly onto printed circuit boards. The singulation of the semiconductor packages result in contacts on peripheral edges of the singulated semiconductor packages.
FIG. 1 illustrates a singulated semiconductor package 100 having a plurality of contacts 105 on peripheral edges of the singulated semiconductor package. Although the bottoms of the contacts 105a are tin plated, sidewalls 105b of the contacts 105b are exposed (e.g., without tin plating), because the singulation occurred after the semiconductor array was electroplated with the lead finished material. If the singulated semiconductor package 100 is stored in inappropriate environments and/or conditions (e.g., moisture in the air, acids, bases, salts, oils, aggressive metal polished, and other solid and liquid chemicals) after singulation, then the exposed surfaces 105b become sites for potential corrosion 110 such as copper oxide. This aging process is known as oxidation. The exposed surfaces 105b, usually deposited with pollutant layers of oxide and other nonmetallic compound 110, often interfere with or inhibit solder wettability. The resulting oxide layer reduces solderability because contamination 110 prevents the metal from soldering well. The rate of oxidation can increase with an increase in temperature or humidity. Solder problems are a common cause for device failures.
A perfectly clean surface is required for assembly of the singulated semiconductor packages 100 onto a substrate, such as a printed circuit board. Since metal oxides form a barrier that prevents molten solder from forming a true metallurgical bond, the metal oxides must be limited.